1982 430 WR starting build, ADVICE please!

I tapped the hole in a WC cover and put a 3/8" NPT plug in it. Worked great! Tried a soft plug but it didn't cover the little hole.

Think I'll make a cover plate, Hate to ruin a cover cause I might need it for the next water cooled project!
Besides maybe I can use milling machine at work (government job if you will)and make a nice little cover with some fins and a gunsight logo since I work so many hours on the back shift?

Hi, I'Ve got an 82 430 XC starting a raceable resto, the left center case is broken. Will the cases from the 250 of the same year work? Thanks.

I don't know? Maybe one of the Guru's will help? I'm not sure.